1. Field of the Invention
This invention relates to a positive resist composition which is usable in the course of producing semiconductors such as ICs, producing liquid crystals or circuit boards such as thermal heads and lithographing other photoapplications, and a pattern making method using the same. In particular, it relates to a positive resist composition appropriately usable in exposure with the use of an immersion type projection exposure device wherein far-ultraviolet light having a wavelength of 300 nm or less is employed as the light source, and a pattern making method using the same.
2. Description of the Related Art
With the recent fine patterning in semiconductors, attempts have been made to shorten the wavelength of an exposure light source and elevate the numerical aperture (high NA) of a projector lens. At present, there has been developed an exposure device with NA 0.84 using an ArF excimer laser beam having a wavelength of 193 nm as a light source. It is widely known that such devices can be indicated by the following formulae.(Resolution)=k1(λ/NA)(Focal depth)=±k2λ/NA2 
In the above formulae, λ represents the wavelength of an exposure light source; NA represents the numerical aperture of a projector lens; and k1 and k2 represent coefficients relating to the process.
To further shorten the wavelength and elevate the resolution, studies have been made to employ an exposure device with the use of an F2 excimer laser beam having a wavelength of 157 nm as a light source. In this case, however, the lens material and the resist material to be used in the exposure device for shortening the wavelength are highly restricted. Thus, it becomes very difficult to stabilize the production costs of the device and the materials as well as the qualities thereof. As a result, it is feared that an exposure device and a resist having sufficient performance and stability could not be obtained on time.
As a technique for elevating the resolution of an optical microscope, there has been known a so-called immersion method wherein the space between a projector lens and a sample is filled with a liquid having a high refractive index (hereinafter also called “immersion liquid”).
Concerning this “immersion effect”, the resolution and focal depth as described above can be indicated by the following formulae, wherein λ0 represents the wavelength of the exposure light in air, n represents the air refractive index of the immersion liquid, θ represents the convergence half angle of the light, and NA0 is referred to as sinθ.(Resolution)=k1(λ0/n)NA0 (Focal depth)=±k2(λ0/n)NA02 
Namely, the immersion effect is equivalent to using exposure light of the wavelength 1/n. In other words, the immersion makes it possible to elevate n-fold the focal depth in the case of using a projection optical system of the same NA. This is effective on any patterns and, moreover, can be combined with the super-resolution techniques under study, for example, the phase-shift method and the distortion illumination method.
An example of a device in which the above effect is applied to the minute pattern transfer is proposed by, for example, JP-A-57-153433.
Recent advances in the immersion exposure technique are reported by SPIE Proc 4688, 11 (2002), J. Vac. Sci. Technol. B 17 (1999), SPIE Proc 3999, 2 (2000) and so on. In the case of using an ArF excimer laser beam as a light source, it is considered that purified water is the most desirable immersion liquid from the viewpoints of safety in handling, the transmittance and refractive index at 193 nm (refractive index at 193 nm: 1.44). In the case of using an F2 excimer laser beam as a light source, attempts have been made to use a fluorine-containing solution by taking the balance between the transmittance and refractive index at 157 nm into consideration, though no sufficient liquid has been found out so far from the viewpoints the environmental safety and refractive index. Based on the extent of the immersion effect and the achievement level of resists, it is expected that the immersion exposure technique will be applied first to ArF exposure devices.
After the development of a resist for a KrF excimer laser beam (248 nm), use has been made a so-called chemical amplification image forming method that is a resist image forming method for compensating for lowering in sensitivity caused by light absorption. In the positive chemical amplification method for forming an image, for example, an acid-generating agent is decomposed upon exposure in an exposed part to form an acid. In baking after the exposure (PEB: post exposure bake), the acid thus generated is utilized as a reaction catalyst and thus an alkali-insoluble group is converted into an alkali-soluble group. Thus, the exposed part is removed by the alkali development to thereby form an image. As resist compositions of the chemical amplification type, for example, resist compositions comprising a mixture of two or more resins having definite structures are proposed by WO 2005/003198 and JP-A-2002-303978. Although ArF excimer resists using the chemical amplification mechanism have become mainstream in these days, they suffer from a problem that a pattern becomes misshapen when exposed at an ultrafine mask size. Thus, it has been required to overcome this problem.
When a chemical amplification resist is applied to immersion exposure, the resist layer comes into contact with an immersion liquid at the exposure. It is pointed out that, in this step, the resist layer is denatured and a component exerting an undesirable effect on the immersion liquid oozes from the resist layer. According to WO 2004-068242, when a resist for ArF exposure is immersed in water before and after the exposure, the resist performance alters, which is pointed out as a problem in the immersion exposure.
In the case of using a scanning type immersion exposure machine in the immersion exposure process, it is feared that the exposure speed is lowered and thus the productivity is adversely affected unless the immersion liquid follows up the movement of a lens. When water is employed as the immersion liquid, it is required that a resist film has a hydrophobic nature and well follows-up to water.